When studying alkene reactions in organic chemistry, you will often come across a mechanism that requires you to place two halogen atoms on an alkene. Understanding the mechanism will help you visualize the products and allow you to perform this reaction flawlessly on your upcoming quiz or exam. In this article I will help you understand that what and how of the halogenation mechanism
This mechanism, as the name implies is related to halogens or halides. These atoms are of particular importance to organic chemistry due to their high electronegativity and reactivity. You will particularly see the halogenation mechanism carried out using a dibromine or dichlorine added across a carbon to carbon double bond. This results in a halogenated product contained two neighboring halogens
Since this is an electrophilic alkene addition, it is important to understand the nature and reactivity of each of the reactants. The alkene, with its negative pi electrons is considered the nucleophile in this reaction. The dihalide, while non-polar to begin with, will obtain and induced polarity when in proximity to the alkene. The partially positive halide, while temporary, will be the electrophile in this reaction
The electrophilic halogen will induce the nucleophilic pi bond to reach out thereby breaking the carbon to carbon double bond. The alkyl chain now has one carbon atom bound to a halogen, and the second carbon atom with a positive charge. The intial halogen bond is also broken in this attack, leaving one halogen bound to the carbon atom, and the second halogen remains as a halide in solution with a negative charge
The Halogen that is attached to one of the carbon atoms is still quite electronegative. It will therefor use one of its lone electron pairs to reach out to the carbocation and form a second bond. Since the halogen is now bound to both carbons it forms a halogen bridge. This in turn places a positive charge on the halogen.
The halogen bridge is very unstable for a number of reasons. Halogens, being very electronegative do not like having a positive charge. And since the bridge is formed between three atoms, the bond angle will be quite constrained at near sixty degrees in place of the favored 109.5 degree angle for an sp3 hybridized atom. This instability invites the negative halide to attack at one of the carbon atoms breaking the bridge and resulting in the desired dihalide
This mechanism, as the name implies is related to halogens or halides. These atoms are of particular importance to organic chemistry due to their high electronegativity and reactivity. You will particularly see the halogenation mechanism carried out using a dibromine or dichlorine added across a carbon to carbon double bond. This results in a halogenated product contained two neighboring halogens
Since this is an electrophilic alkene addition, it is important to understand the nature and reactivity of each of the reactants. The alkene, with its negative pi electrons is considered the nucleophile in this reaction. The dihalide, while non-polar to begin with, will obtain and induced polarity when in proximity to the alkene. The partially positive halide, while temporary, will be the electrophile in this reaction
The electrophilic halogen will induce the nucleophilic pi bond to reach out thereby breaking the carbon to carbon double bond. The alkyl chain now has one carbon atom bound to a halogen, and the second carbon atom with a positive charge. The intial halogen bond is also broken in this attack, leaving one halogen bound to the carbon atom, and the second halogen remains as a halide in solution with a negative charge
The Halogen that is attached to one of the carbon atoms is still quite electronegative. It will therefor use one of its lone electron pairs to reach out to the carbocation and form a second bond. Since the halogen is now bound to both carbons it forms a halogen bridge. This in turn places a positive charge on the halogen.
The halogen bridge is very unstable for a number of reasons. Halogens, being very electronegative do not like having a positive charge. And since the bridge is formed between three atoms, the bond angle will be quite constrained at near sixty degrees in place of the favored 109.5 degree angle for an sp3 hybridized atom. This instability invites the negative halide to attack at one of the carbon atoms breaking the bridge and resulting in the desired dihalide
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To read about this mechanism in greater detail, visit this page about Alkene Halogenation Mechanism
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